The apparatus and method of the present invention relate, in general, to the field of the formation of threads on a cylindrical workpiece, and more particularly, they relate to the formation of a tapered thread on a concrete reinforcing bar.
There are many applications in which concrete reinforcing bars must be joined in end-to-end relation. These bars are usually formed with a pattern of ribs on their exterior so that when they are buried in the concrete they are effectively coupled to the concrete in a manner which will permit tension loads to be supported by the reinforcing bars.
For many years the coupling together of reinforcing bar ends was accomplished by welding or overlapping of sufficient lengths of the bars so that tension forces were effectively transmitted over the joint. Overlapped bars were usually wired together for casting purposes. The ribs on reinforcing bar ("rebar") made it difficult to mechanically couple rebar together in end-to-end relation. One approach to such mechanical coupling was to employ a coupling sleeve in which a pyrotechnically melted matrix of metal was used to lock the rebar ribs to the grooves inside the coupling sleeve. This system was commercially exploited under the trademark CADWELD. Another system for mechanical coupling employed a taper-threaded rebar coupler, namely, the coupler set forth in U.S. Pat. No. 3,415,552 and sold under the trademark FOX-HOWLETT NO-SLIP. This coupler required that the ends of the reinforcing bar be formed with a tapered thread so as to permit a coupling sleeve having mating taper-threaded ends to be used to form a high strength mechanical splice between the two reinforcing bars.
The mechanical coupler of U.S. Pat. No. 3,415,552 has now become the rebar coupler of choice in most projects, replacing prior welding, thermite and other bar joining techniques in the industry. One of the several advantages of such mechanical couplers is that four to five rotations of the coupling sleeve with respect to the rebar results in a complete threading together of the tapered threads over the entire length of the threads. For applications in which the reinforcing bar cannot be rotated, a taper-threaded union assembly also has been developed, as is set forth in U.S. Pat. No. 3,850,535.
While the taper-threaded reinforcing bar splice is a highly effective coupling system, the problems in connection with forming a tapered thread on the end of a ribbed concrete reinforcing bar are substantial, particularly as the size of the reinforcing bar increases. A No. 18 reinforcing bar, for example, weighs 14 pounds per foot, making manipulation of longer lengths of the bar a significant problem. Additionally, the ribs on the exterior of reinforcing bars pose machining problems that can be substantial.
Two approaches have been taken to the cutting of tapered threads on the ends of ribbed rebar. The first has been to modify a thread milling machine to enable formation of the tapered thread. Thread milling machines include a mechanism for slowly rotating the workpiece as the cutting tool is advanced.
Canadian Pat. No. 1,034,307 sets forth in detail a thread milling machine suitable for use in forming tapered threads on a rebar. Briefly, a milling head is set up at the proper angle, usually about 6 degrees, and then gradually advanced into the bar until the milling head has reached the proper depth in the end of the bar. The bar is then slowly rotated and simultaneously the milling head is advanced at an angle with respect to the bar's longitudinal axis so that a tapered thread having a lead is formed on the bar end. The primary disadvantage of this approach has been the size of the machine required and the need to rotate the heavy and unwieldy reinforcing bar, even though the rotation is slow and only through one revolution. Such taper-thread cutting apparatus do have the advantage of being able to form the tapered thread by a single pass or cutting process.
The other approach employed in the industry has been to form a rebar taper-thread cutting machine from a heavy-duty pipe threading machine. In such pipe threaders, the bar can remain stationary and thread chasers, held in the cutting head, are simultaneously rotated, displaced along the bar at an angle to form the tapered thread. The lead for the thread is produced by the chasers themselves, which tend to automatically feed axially along the bar. The thread chasers are used not only to form the thread, but also act to remove the necessary material from the end of the bar to produce the taper. Thus, the bar is not tapered by another tool and then threaded, but instead, the chasers are both removing material and threading. Since the amount of material to be removed is substantial on the larger bar sizes, two to four passes by the chasers are made in order to form the tapered thread at the desired depth or small-end diameter.
Several disadvantages have been encountered in connection with the use of heavy-duty pipe threaders for the formation of tapered threads on reinforcing bars. First, the bar itself is not rigidly clamped in the pipe threader, but is held in a self-aligning vise. This permits movement of the bar relative to the cutting head, with the result that ribs on the bar can influence the orientation of the frusto-conical thread. Additionally, since the apparatus has no positive lead control, the lead which is automatically cut by the thread chasers is sometimes imprecise. Both of these problems can often be accommodated by the taper-threaded coupling sleeve as long as they are within a range of tolerances.
Prior pipe threading machines also require that the end of the reinforcing bar be relatively clean and square to the longitudinal axis of the rebar. Reinforcing bars, however, are often cut to length by a shear, which can deform the bar end or leave a burr or uneven surface on the bar end. Thus, rebar fabrication shops often have to prepare the bar ends for threading in heavy-duty pipe threaders by sawing the bar end. This is a serious problem in terms of the need for a time consuming preparatory operation and the need for sawing equipment which is often not available in rebar fabrication shops.
Another serious problem in connection with heavy-duty pipe threading apparatus is that the cutting head and thread chasers surround the bar end so that the mechanism for radially displacing and advancing the thread chasers is exposed to chips cut from the bar. Since part of the process is to remove a significant amount of material from the bar end, chips not infrequently become lodged in the head guiding and control mechanism, which necessitates an undesirable amount of maintenance and repair. The thread cutting process normally is facilitated by the use of liquid coolant, which tends to carry the bulk of the chips out of the head, but the coolant also distributes chips to the surrounding guide surfaces of the cutting head so as to constantly expose the same to debris.
Another consideration in the formation of taper-threaded rebar coupling splices is the cost of bar threading. Using the modified thread milling machine, the cost of threading bar ends ranges from between about $2.75 to about $5.60 per end, including machine costs, cutter sharpening and replacement costs, and the labor cost. For a heavy-duty pipe threader, the costs are believed to be less for the smaller size bars, but they become more for No. 11 to No. 18 bars. The increase in cost is a result of the increase in the amount of material which must be removed and the attendant increase in labor and maintenance costs.